In a reciprocating pump, drive power of a drive-power supply member is transmitted to a diaphragm within a diaphragm drive chamber via operating fluid. An operating-fluid-flow regulation chamber for regulating the flow of operating fluid is located between the drive-power supply member and the diaphragm drive chamber. A first gas discharge member and a second gas discharge member are respectively located in an upper part of the inside of the operating-fluid-flow regulation chamber and an upper part of the inside of the diaphragm drive chamber. The first gas discharge member is held in fluid communication with the second gas discharge member so as to constitute a single gas discharge mechanism. This single gas discharge mechanism is provided with a reverse-flow prevention member for preventing a reverse flow of fluid from the first gas discharge member to the second gas discharge member.